1. Field of the Invention
This invention pertains to the field of microwave antennas. More particularly, this invention pertains to a dual frequency reflector feed element for parabolic antennas that simultaneously operates in two separate frequency ranges such as the C-band and the Ku-band.
2. Description of the Prior Art
It is well-known in the field of antennas that the feed element for a reflector-type antenna must provide a radiation pattern that properly illuminates the reflector. In particular, the peak of the illuminating pattern is ordinarily directed at the center of the reflector and the amplitude of the pattern falls off with angle such that the amplitude directed toward the edge of the reflector is approximately 10 dB below the peak amplitude at the center of the reflector. This results in what is commonly regarded as an optimum radiation pattern for the reflector antenna in terms of gain, beam width and side lobe level.
A uniform illumination would result in a narrower beam width but also in unwanted higher side lobe levels. The higher side lobe levels increase the noise figure of the antenna system which degrades performance. A greater amplitude taper, such that the amplitude directed toward the edge of the reflector being more than 10 dB below the peak amplitude directed at the center of the reflector, will reduce side lobe levels but increase beam width and reduce gain in the antenna. The reduction in gain also has the effect of increasing the noise figure of the whole antenna system. Other non-uniform illumination patterns generally result in reduced gain and/or increased side lobe levels with the consequent reduction in overall system sensitivity.
Since the most common reflector configuration is circular, the E-plane and H-plane radiation patterns of the feed must be of very nearly the same beam width. A common feed element utilizes a dipole element which has an omni-directional H-plane pattern. In order to provide a proper illuminating pattern, the dipole is mounted in front of a conductive cavity and located approximately 0.25 wavelengths (.lambda.) above the conductive floor of the cavity. When thus configured the cavity constrains the feed element radiation to the correct direction with respect to the reflector.
The spacing between the dipole antenna and the floor of the cavity results in the reflection of radiated energy in the direction of the reflector such that the reflected radiation is in phase with the direct radiation and adds constructively, resulting in an increase in gain of the feed of about 3 dB. This is highly desirable since it improves the efficiency and sensitivity of the whole system. However, if the cavity is 0.25 wavelengths deep at the lowest frequency band, the radiation patterns at the high band will be poorly shaped and lobey because the energy reflected from the cavity floor will generally not be in phase with the direct radiation and system gain will be reduced at the high band. Similarly, if the cavity is 0.25 wavelengths deep at the high frequency band the radiation patterns at the low frequency band will be degraded with a consequent reduction in system gain at the low band.
In the area of direct broadcast via satellite of television signals to homes, it is desirable for the satellite receiving antenna to receive efficiently at two distinct bands since the satellites that broadcast television signals transmit at two different frequency bands. The bands are presently identified as "C-Band" which consists of signals transmitted at a frequency between 3.7 GHz and 4.2 GHz, and "Ku-Band" which consists of signals transmitted at a frequency between about 10.6 GHz and 12.7 GHz.
In the present state of the art, symbolized by patents such as U.S. Pat. Nos. 4,740,795; 4,872,211; 4,903,037; 5,066,958; 5,107,274; and, 5,255,003, feeds with the capability of receiving both C-Band and Ku-Band signals typically consist of two separate antennas merged into a single complex and consequently expensive assembly. In a common design, the low band antenna consists of an open ended wave guide or cavity with a probe near the bottom thereof that picks up the received signal and conducts it to another section of the wave guide which conveys the signal to an integral low noise amplifier and down converter. The high band antenna is a yagi-type antenna consisting of a dipole, a corner reflector behind the dipole and a passive dipole in front of the dipole. This high frequency antenna is mounted in the mouth of the low band wave guide and the coaxial cable from the high band dipole is lead down through the wave guide along its centerline and through the floor to a third wave guide which conveys the high band signal to a second low noise amplifier and down converter.
Accordingly, there is a need in the art for a simple, inexpensive feed element that properly illuminates a reflector at two separate frequency bands such that optimum system performance is achieved at both bands.